A mobile station, also known as a User Equipment (UE), wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication network, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between user equipment, between a user equipment and a wire connected telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks. The wireless communication may comprise various communication services such as voice, messaging, packet data, video, broadcast, etc.
The mobile station may further be referred to as mobile telephone, cellular telephone, computer tablet or laptop with wireless capability, etc. The mobile station in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station, a stationary entity or a server.
The wireless communication network covers a geographical area which is divided into cell areas, with each cell area being served by a network node, radio network node or base station, e.g., a Radio Base Station (RBS) or Base Transceiver Station (BTS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and/or terminology used.
Sometimes, the expression “cell” may be used for denoting the network node itself. However, the cell may also in normal terminology be used for the geographical area where radio coverage is provided by the network node at a base station site. One network node, situated on the base station site, may serve one or several cells. The network nodes may communicate over the air interface operating on radio frequencies with any mobile station within range of the respective network node.
In some radio access networks, several network nodes may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks. LTE is based on the GSM/EDGE and UMTS/HSPA network technologies, increasing the capacity and speed using a different radio interface together with core network improvements.
LTE-Advanced, i.e. LTE Release10 and later releases are set to provide higher bitrates in a cost efficient way and, at the same time, completely fulfil the requirements set by International Telecommunication Union (ITU) for the International Mobile Telecommunications (IMT)-Advanced, also referred to as 4G.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the network node to the mobile station. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the mobile station to the network node.
Densification of radio network nodes is expected to lead to the spectral efficiency requirements envisioned for future radio access networks. However studies have showed that a plain network densification would significantly increase the overall energy costs. Therefore, future generations of dense radio access network should be co-designed to be both spectral- and energy-efficient.
While network nodes in traditional 4G radio cellular systems have been proven to transmit user data-plane about 5% of the subframes, this number may be significantly lower in denser networks, e.g. as low as 1% or less. Therefore, to fulfil both spectral- and energy-efficiency requirements, network nodes in dense networks can be dynamically switched on/off network nodes to follow traffic variations or other relevant network statistics. More practically, the network nodes shall operate in Discontinuous Transmission (DTX) mode, where the network node is active when there is traffic to be served, whilst it transits into a dormant state with limited transmission and reception capabilities otherwise. Thus, with a discontinuous transmission ratio of 1%, for a transmission time of tens to hundreds of milliseconds (e.g., 100 ms) the “off-time” would be in the order of tens of seconds (e.g., 10 s).
Future releases of the related art LTE system, for instance, will adopt this feature for enhanced small cell operation (i.e., network nodes with a small coverage area, sometimes referred to as micro cells, pico cells, nano cells etc.). Typical scenarios may be exemplified by two cases: (a) a macro-cell layer providing large coverage and clusters of small-cell nodes deployed in the same frequency band as the macro-cell layer (i.e., co-channel deployment) or in a disjoint frequency band (i.e. non-co-channel deployment); and (b) isolated clusters of small-cells without macro-coverage. In the first case, the macro-cell (a.k.a. primary cell (P-cell) or serving cell in LTE terminology) may provide network assistance to the mobile stations for operating in the small-cell layer (a.k.a. secondary cells (S-cells)). In the second case, network assistance may be provided by a cluster head as well as coordination among the small-cells operation.
Enabling network nodes to operate with long DTX cycles with very short active time has severe consequences in the efficiency of the cell discovery procedures and the relative energy consumption at the mobile station. The cell-search procedure of the related art 3GPP LTE system, for instance, is designed for a sparse deployment of macro base stations (eNodeB) assumed constantly active. The procedure comprises a cell detection step, upon which a mobile station acquires time and frequency synchronization, followed by a phase in which the mobile station acquires other crucial system parameters that are necessary to demodulate other downlink signals. Cell detection comprises a synchronization step based on Primary and Secondary Synchronization Signals (PSS/SSS), followed by a measurement step based on downlink Common Reference Signals (CRS) used to verify the cell ID and perform initial signal strength measurements. These measurements are performed by mobile stations in IDLE mode as part of the cell-search procedure, as well as by mobile stations in RRC_CONNECTED mode when required to monitor, by the serving cell, a set of neighbouring cells for handover purposes.
The requirements for cell detection, listed in Table 1, combine a minimum Signal to Interference+Noise Ratio (SINR) condition with a maximum allowed detection time, e.g., (−6 dB, 800 ms) for intra-frequency with a measurement period of 200 ms. The measurement sampling is implementation specific, but typical values range in 1-2 ms sample/snapshot per 40 ms or per discontinuous reception (DRX) cycle.
TABLE 1E-UTRANE-UTRANintra-frequencyinter-frequencyReceivedCRS: Es/IoT≥−6 dBCRS: Es/IoT≥−4 dBsignalPSS/SSS: Es/IoT≥−6 dBPSS/SSS: Es/IoT≥−4 dBqualityMeasuredUp to 7 cellsUp to 3 inter-frequencies andcellsup to 4 cells per frequencyTimeDetection time:Detection time: (3.84 s,requirements800 ms7.68 s) · NfreqMeasurement period:Measurement period:200 ms480 ms · Nfreq
With the introduction of a DTX operation for network nodes, some or all the signals that are typically transmitted to aid the mobile station to detect a network node, synchronize to, measure the signal strength, and access the network may either be absent or transmitted only sporadically. Detecting the presence of a dormant cell may therefore require longer monitoring time, thereby draining the battery of the mobile station. On the other hand, the prior art procedures are not sufficiently fast to enable a mobile station to quickly detect and perform signal strength measurements from a network node when it operates with discontinuous transmission followed by a very short active time. Therefore, new solutions are needed to aid the mobile stations determine not only the presence of a dormant network node, but also to avoid inefficient usage of battery at the mobile station when the network node operates in DTX mode.
When network nodes in a communication system are enabled to operate with a discontinuous transmission (DTX) mode, i.e., a long dormant state with limited transmission/reception capabilities followed by a short active time, it is a problem to assure fast and energy-efficient detection of the cell at the mobile station. A mobile station failing to detect any network node may need to extend the search procedure for (at least) as long as the shorter reactivation cycle among the dormant cells in its proximity. A further problem is related to timely signal strength measurements such as i.e., Radio Resource Management (RRM) measurements and/or Radio Link Management (RLM) measurements, for network nodes in DTX state. In particular, when a mobile station is configured to monitor the signal strength of network nodes that operate in DTX mode, the mobile station might unnecessarily consume energy if the measurements are taken too early and/or too often prior the network node comes back on active state.
Therefore, it is a problem of the previously known related art to assure timely and energy-efficient cell detection and RRM/RLM measurements at the mobile station when network nodes can operate in a low-duty cycle discontinuous transmission mode.
In the related art 3GPP LTE-Advanced system, a network node in the dormant state of a low-duty cycle discontinuous transmission (DTX) mode shall transmit Discovery Reference Signals (DRS) to aid mobile stations in its proximity to detect its presence and measure signals strength. Discovery signals in the 3GPP LTE-A have been studied in relation to two problems: 1) enhancing the detection performance of active cells in denser small cell deployments; and 2) enabling the discovery of small cells in a dormant state (a.k.a. “off-state”). The requirements and solutions for detecting a small cell in these two cases, however, may be rather different. In the first case, active cells are expected to continue using at least the legacy PSS/SSS/CRS signals and the performance of the cell-search procedure can be improved, for instance, through methods based on PSS/SSS interference cancellation. For the second case, however, the main objective is to enable the mobile station to detect the presence of a cell in a dormant state and perform RRM measurements. Energy-efficiency of the small cell discovery at the mobile station is a crucial aspect in both cases and it should therefore be taken into account in the design of discovery signals.
Discovery signals transmitted by network nodes during a DTX period consist of short bursty transmissions of downlink signals that the mobile station can detect to infer the presence of a dormant network node. The 3GPP TSG-RAN WG1 is currently considering a DRS design for the related art LTE-A based on periodic bursts of existing synchronization signals (i.e., PSS/SSS), existing reference signals such as Channel State Information Reference Signals (CSI-RS), Positioning Reference Signals (PRS), modified version of the LTE synchronization signals and reference signals in LTE, or a combination thereof.
For instance, CSI-RS and PRS have been considered for the design of synchronized transmission of discovery signals in clusters of cells. One method is to let each small cell in a cluster transmit a CSI-RS of a different configuration pattern while muting the CSI-RS resources for all other configuration patterns, thereby enabling fully orthogonal DRS within a cluster. Another method is to let a small cell transmit PRS signals with sub-carrier shifts of reuse factor 6 according to the Physical Cell ID (PCI). Either way, a mobile station may have an advantage of network assistance to acquire the signal's configuration, e.g., carrier, bandwidth, and time-frequency resource used by each cell.
Mobile stations with a connection to an active serving cell, referred to as RRC_CONNECTED mobile stations in LTE, are typically required to monitor reference signals of a set of neighbouring cells to perform RRM/RLM measurements for facilitating the handover procedure. For RRC_CONNECTED mobile stations, the serving network node may provide further assistance to detect the presence of other network nodes in DTX mode. In one example, the serving network node may provide the rough timing of a cluster of synchronized small cells within its coverage area, so that the mobile station can reduce the effort on cell-detection by avoiding synchronizing to the small cell cluster. The serving network node may further inform the mobile station the PCI-like information necessary to identify the neighbouring network nodes in dormant state the mobile station is required to monitor, as well as information related to the configuration of their reference signals.
In contrast, mobile stations in RRC_IDLE mode do not belong to a specific cell and no data transfer my take place prior a connection is formally established with a cell. In this case, network assistance cannot be provided to help the mobile station detect the presence of network nodes in a dormant state. With the legacy LTE cell-search procedures, a mobile station that cannot detect an active cell would persist in the cell-search for (at least) as long as the shorter reactivation cycle among the dormant network node in the mobile station proximity. As this could amount to tens of seconds, the cell-search would severely drain the mobile station battery.
The known prior art, however, cannot assure timely and energy-efficient cell detection and RRM/RLM measurements for either RRC_IDLE and/or RRC_CONNECTED mobile stations. Therefore, from an energy-efficiency view point, it is desirable to introduce new mechanisms for assisting the mobile station.